Semiconductor laser with integral spatial mode filter

ABSTRACT

A semiconductor laser having a light amplifying diode heterostructure body having a single spatial mode aperture region or waveguide and a flared or tapered gain region having a narrow input end and wider output end provided in a resonant cavity, a portion of which cavity may be external of the body. The flared gain region has a narrow aperture end and a wide output end with narrow aperture end optically coupled to a single mode waveguide. A saturable aborbing region is formed as part of the single mode waveguide region and not between it and the flared gain section, and is reverse biased to provide for mode locked operation. The flared gain region and waveguide may be differentially pumped or modulated with current provided by separate contacts, and the flared gain region may be divided into one or more flared gain sections which may be differentially or separately pumped.

This is a continuation of application Ser. No. 08/483,667 filed Jun. 7,1995, now abandoned which is a division of application Ser. No.08/263,190 filed on Jun. 21, 1994, now U.S. Pat. No. 5,592,503 which isa division of Ser. No. 08/001,735 filed on Jan. 7, 1993 now U.S. Pat.No. 5,392,308.

TECHNICAL FIELD

The invention relates to external-cavity semiconductor lasers,especially to those lasers that include a frequency-selective tuningelement for broadband tunability and narrow linewidth light emission.The invention also relates to lasers with single spatial mode,diffraction-limited emission, and to light amplifying diodeheterostructures with flared gain regions.

BACKGROUND ART

External-cavity semiconductor lasers, including lasers with frequencyselective tuning elements in the cavity, are well known and have beenextensively studied. For example, T. Fujita, et al., in Applied PhysicsLetters 51(6), pages 392-394 (1987), describe a laser having a buriedheterostructure laser that has been antireflection (AR) coated on theintracavity facet, a collimating lens, a polarization beamsplitter,external cavity mirrors in each of the TE and TM polarization lightpaths, and an electro-optic modulator in the TE polarization pathbetween the beamsplitter and cavity mirror. The configuration allowsselection of either the TIE or TM mode of oscillation by adjusting themodulator's bias voltage. W. Sorin, et al., in Optics Letters 13(9),pages 731-733 (1988), describe a laser having a laser diode with one ofits facets AR coated to reduce its reflectivity, a lens, a single modeoptical fiber and a tunable evanescent grating reflector for providingfeedback. The laser is wavelength tunable by sliding the feedbackgrating laterally over the fiber. P. Zorabedian et al., in OpticsLetters 13(10), pages 826-828 (1988), describe another wavelengthtunable laser using either a rotatable interference filter in anexternal Fabry-Perot cavity or an external grating reflector providingfrequency-selective feedback.

A problem with previously available external-cavity semiconductor lasersis their generally low output power (on the order of 10 mW cw and200-300 m W pulsed). Further, higher output powers are associated withunstable output intensity and frequency and less than good modalquality.

In U.S. Pat. No. 4,251,780, Scifres et al. describe semiconductorinjection lasers that are provided with a stripe offset geometry inorder to enhance and stabilize operation in the lowest order orfundamental transverse mode. In one configuration, the stripe geometryhas a horn shaped or trapezoidal section connected to a straightsection, in which the width of the horn shaped or trapezoidal sectionexpands from 8 μm at the straight section to 25 μm at the cleaved endfacet. In contrast to configurations in which the edges of the stripewaveguides are linear and orthogonal to the cleaved end facets of thelasers, the nonorthogonal angled or curved edges of the offset stripegeometries cause higher order modes to reflect or radiate out of thewaveguide, thereby increasing the threshold of the higher order modesrelative to the fundamental mode.

In U.S. Pat. No. 4,815,084, Scifres et al. describe semiconductor lasersand laser arrays in which lenses and other optical elements have beenintegrated into the semiconductor bodies of the lasers by means ofrefractive index changes at boundaries in the light guiding region,where the boundaries are characterized by a lateral geometric contourcorresponding to surfaces of selected optical elements so as to causechanges in shape of phase fronts of lightwaves propagating across theboundaries in a manner analogous to the change produced by the opticalelements. In one embodiment, a biconcave or plano-concave diverging lenselement is integrated within the laser in order to counteract theself-focusing that usually occurs in broad area lasers and that can leadto optical filamentation and lateral incoherence across the laser. Thediverging lens in the laser allows the laser to operate as an unstableresonator, leading to high output power and good coherence across thelateral wavefront.

An object of the invention is to provide a high power, external cavity,semiconductor laser which emits a single spatial mode,diffraction-limited output beam.

Another object of the invention is to provide a wavelength tunable, highpower, external cavity, semiconductor laser with a stable, singlefrequency, narrow linewidth light output.

DISCLOSURE OF THE INVENTION

The above objects are met with a laser in which a semiconductor activemedium is located within an at most marginally stable resonant cavitywith a single-spatial-mode filter therein. The semiconductor activemedium is preferably an electrically pumped light amplifying diodeheterostructure or "amplifier chip" that has a flared gain region with anarrow, single mode, optical aperture end and a broad light output end.The flared gain region permits the light to freely diffract as itpropagates in the gain region, so the light has a diverging phase front.Only the central-most light rays of backward propagating light can passthrough the narrow aperture end of the flared gain region to reach anexternal rear reflector of the resonant cavity. Rear reflectors integralwith the diode heterostructure could also be used. The rear reflectorcan be a mirror surface or a frequency selective grating reflector.Orientation of the grating reflector determines which wavelength oflight will couple back through the narrow aperture in the amplifier chipinto the flared gain region. The flared gain region ensures high poweramplification of forward propagating light while maintaining a singlespatial mode of oscillation.

The invention also includes related master oscillator power amplifier(MOPA) devices in which a first portion of the above describedsemiconductor active medium is located within the resonant cavity toform a laser oscillator with external rear reflector, while a secondportion of the same active medium is located outside the resonant cavityto form an optical power amplifier that is optically coupled to thelaser oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top plan view of a wavelength tunable, externalcavity, semiconductor laser of the present invention.

FIG. 2 is a schematic side view of another wavelength tunable, externalcavity, semiconductor laser of the present invention.

FIGS. 3A and 3B are respective top and side plan views of the wavelengthtunable, external cavity, semiconductor MOPA device of the presentinvention.

FIG. 4 is a schematic top plan view of a broadband tunable, externalcavity, semiconductor MOPA device of the present invention.

FIG. 5 is a schematic top plan view of yet another external cavitysemiconductor laser embodiment of the present invention.

FIGS. 6A-6H are top plan views of eight possible light amplifying diodeheterostructures or "amplifier chips" for use in the laser and MOPAembodiments of FIGS. 1-5.

FIG. 7 is a top plan view of yet another amplifier chip for use in thelaser and MOPA embodiments of FIGS. 1-5.

FIG. 8 is a side sectional view taken along the line 8--8 in FIG. 7.

FIG. 9 is a top plan view of still another amplifier chip for use in thelaser and MOPA embodiments of FIGS. 1-5.

FIG. 10 is a side view of an alternate external cavity semiconductorlaser of the present invention with vertical output.

FIG. 11 is a top plan view of another external cavity semiconductorlaser embodiment of the present invention.

FIG. 12 is a side view of yet another external cavity semiconductorlaser of the present invention.

FIG. 13 is a perspective view of an alternative amplifier chip for usein lasers of the present invention.

FIG. 14 is a side schematic view of an external cavity laser using theamplifier chip of FIG. 13.

FIG. 15 is a perspective view of a monolithic array amplifier chip ofthe present invention.

FIG. 16 is a top plan view of a laser array using the amplifier chip ofFIG. 15.

FIG. 17 is a top plan view of a frequency switchable laser embodiment ofthe present invention.

FIGS. 18-24A and 24B are top plan views of alternate external cavitylasers of the present invention.

FIGS. 25A and 25B are respective top and side plan views of a wavelengthtunable, external cavity, semiconductor laser of the present inventionwith differential pumping.

FIG. 26 is a schematic top plan view of an external cavity MOPAembodiment of the present invention with a tunable grating integrated onthe amplifier chip of the MOPA device.

FIGS. 27-32 are top plan views of additional semiconductor lasers of thepresent invention with an integral spatial mode filter and integralcavity reflectors.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, an external-cavity semiconductor laser, inaccord with one embodiment of the present invention, has an active gainmedium that is a light amplifying diode heterostructure or "amplifierchip" 11, and also has a light reflective, external, diffraction grating15 and a lens 13. The amplifier chip 11 shown in FIG. 1 has a singlemode waveguide section 17 incorporated on the grating side of the chip,opening into a flared gain section 19 on the output side of the chip.Preferably, the flared gain region 19 is linearly flared and increasesin width toward the front output facet 21 of the amplifier chip 11 at arate that is slightly greater than the divergence of light propagatingwithin the flared gain region 19. The front output facet 21 is typicallycoated for low reflection. Though a facet reflectivity of 30% wouldlikely be acceptable, typically the reflectivity of the coated facet 21is less than 10%, with a 2 to 3% reflectivity being preferred. The rearfacet 23 on the grating side of the amplifier chip 11 is antireflectioncoated in order to suppress self-oscillation of the chip. A reflectivityof 1% or less is preferred.

The lens 13 is a high numerical aperture lens positioned to receive andcollimate light emitted from the single-mode waveguide 17 through theantireflection-coated rear facet 23. A spherical lens with a focallength of about 6.5 mm is typical. A graded-index (GRIN) rod lens couldalso be used. Although a simple single element lens is shown, a morecomplex lens system to correct for astigmatic and chromatic aberrationor other optical phenomena could be used.

The front output facet 21 of the amplifier chip 11 and the externalgrating 15 form a frequency-selective optical resonator in whichdiffraction from the reflective grating 15 provides frequency-selectivefeedback of light into the single mode waveguide section 17 of theamplifier chip 11. The wavelength can be tuned by rotating the grating15 about a pivot point 25 until an orientation is reached that coupleslight of the desired wavelength back through the lens 13 and into thesingle mode waveguide section 17. For first order diffraction, thewavelength λ is given by the equation λ=2Λ sin θ, where Λ is the gratingpitch or tooth spacing and θ is the angle of light incidence anddiffraction with respect to the grating normal, as shown in FIG. 1. Atypical diffraction grating for use in the present invention has a linedensity of about 1200 mm⁻¹ (Λ=833 nm) and has a first-order-diffractiondifferential efficiency n₋₁ which is greater than 70%. Small rotationsof the grating 15 can be done with a piezoelectric transducer 27 or by amechanical rotor.

Preferably, the axis of rotation of the grating 15, defined by the pivotpoint 25, will be positioned so that the cavity length is adjusted tocompensate for the change in wavelength as tuning takes place, in orderto minimize mode hopping. This compensation will occur when R=L cos θ₀/tan θ₀, where R is the distance along the grating 15 from the pivotpoint 25 to the optic axis of the cavity, as shown in FIG. 1, θ₀ is theangle of light incidence and diffraction with respect to the gratingnormal for a grating orientation corresponding to a wavelength λ₀ nearthe center of the desired wavelength range, and L is the total opticallength of the cavity at that grating orientation. Then, for smallchanges in orientation (Δθ), the wavelength will tune (Δλ) at the samerate as the longitudinal modes of the external cavity (ΔL=mΔλ, where mis a positive integer). This results in longer continuous tuning ranges,as previously demonstrated by Schremer and Tang in IEEE PhotonicsTechnology Letters, Vol. 2, No. 1, January, 1990, pp. 3-5.

In operation, the single mode waveguide section 17 incorporated in thegrating side of the amplifier chip 11 acts as a spatial mode filter toenhance single spatial mode oscillation in the laser cavity. Also, incombination with the external grating reflector 15, the narrow apertureof the waveguide section 17 acts to select an extremely narrow frequencyband, effectively a single wavelength, for feedback and laseroscillation, since for any given grating orientation, only light of aparticular frequency or wavelength will be diffracted back to theprecise position on the amplifier chip's rear facet 23 needed to coupleinto the narrow waveguide section 17 of the amplifier chip 11. Uponexiting the waveguide section 17 into the flared gain section 19, theforward traveling waves of the light beam are allowed to freely diffractas they propagate in the junction plane of the amplifier chip 11, sincethe flare of the gain section 19 exceeds the divergence of the beam. Thelight has a diverging phase front in the gain section 19, owing at leastin part to the narrow waveguide 17 (Waveguide 17 may be as narrow as 0.5μm to several micrometers wide to cause significant beam diffraction.),and continues to diverge after reflection from the low reflectivityoutput facet 21, as seen in the amplifier chip 11 shown in FIG. 6A. Onlythe central ray 31 returns through the narrow waveguide section 17.Since the low intensity portion of the light beam diverges in travelingback to the narrow end 29 of the gain section, the narrow end 29 andwaveguide 17 act as a spatial filter or aperture to enforce single modeoscillation. Higher order spatial modes experience significantly greaterdiffraction losses within the optical cavity (in addition to havingpoorer overlap with the flared gain element 19)and are thereforesuppressed to high threshold current levels. The external cavity withflared gain section effectively acts as a resonator with a highlyselective spatial filter, which minimizes the net loss of the lowestorder mode relative to higher order modes. It is desirable that opticalpower be output through the low reflectivity facet 21. Alternatively,power output could be obtained by a different order of diffraction offof the grating 15 or by placing a partial beamsplitter in the externalcavity.

In FIG. 1, the external rear grating reflector 15 is oriented so thatthe lines or grooves of the grating, as well as the rotation axis aboutpivot point 25, are perpendicular to the plane of the pn junction in thediode heterostructure 11. However, such an orientation is not essential.FIG. 2 shows an alternate embodiment in which an external rear gratingreflector 34 is oriented so that the lines or grooves of the gratingreflector 34 and the rotation axis about the pivot point 20 are parallelto the plane of the pn junction 26 of the diode heterostructure or"amplifier chip" 22. A resonant optical cavity is defined by the gratingreflector 34 and a lower reflectivity front facet 28 of the amplifierchip 22. The orientation of the grating reflector 34 selects the narrowwavelength band of light that will resonate in the cavity, since onlylight of the selected wavelength will be reflected back upon theincident light path and be imaged by the lens 32 onto the AR coatedintracavity rear facet 30 of the amplifier chip 22 at a position thatwill allow it to be recoupled back into the transverse waveguide 24within the amplifier chip 22. Other wavelengths of light will bediffracted at different angles and so will be imaged by lens 32 eitherabove or below the position of the waveguide 24. The grating orientationshown in FIG. 2 is the more conventional orientation. All figuresshowing an external grating as a portion of the cavity can be used withthe grating orientation of FIGS. 1 or 2.

In FIGS. 1 and 2, the resonant cavity is defined between the externalrear grating reflector 15 or 34 and a front facet reflector 21 or 28 ofthe amplifier chip 11 or 22. Thus, the entire active light amplifyingregion 17 and 19 or 26 is located within the resonant cavity and theresulting device is a semiconductor laser oscillator providing acoherent light output. Alternatively, referring to FIGS. 3A and 3B theresonant cavity can be defined between an external rear gratingreflector 44 and a distributed Bragg reflector (DBR) grating 38 or amicrocleaved or ion milled reflector integrated within the amplifierchip 36. A first, single mode waveguide, portion 40 of the active gainmedium is located within the resonant cavity, while a second, flaredamplifier, portion 50 of the active gain medium is located outside theresonant cavity. The single mode waveguide section 40, DBR grating 38and flared amplifier section are monolithically integrated in amplifierchip 36 with two AR coated end facets. The resulting device is a masteroscillator power amplifier (MOPA) device in which the external gratingreflector 44, a collimating and focusing lens 42, the single modewaveguide section 40 and the DBR grating 38 (or microcleave reflector)form an external cavity laser oscillator and the flared amplifiersection 50 of the amplifier chip 36 forms an optical power amplifieroptically coupled to the laser oscillator. The flared amplifier section50 does not provide substantial feedback to the laser oscillator. Thisdevice can be wavelength tunable, if the external grating 44 can beangularly rotated and if the DBR grating 38 (or microcleave reflector)has a broadband reflectivity. In the case of use of a DBR grating 38 asa cavity reflector, a lower value of the parameter K·L, where K is thecoupling coefficient of the grating to the lightwaves and L is thegrating length, is desirable for a wider tuning range. Alternatively,grating 38 may be electrically tuned in wavelength to match the tuningof external reflector 44, or the reflector 44 may be replaced with aplane mirror and the grating 38 used as the only wavelength tuningmeans.

With reference to FIG. 4, the amplifier chip 52 may have a series ofshort DBR grating segments 58₁, 58₂, . . . , 58_(N) of various gratingpitches Λ₁, Λ₂, . . . , Λ_(N) between the flared amplifier section 54and the single mode waveguide section 56 on the chip 52. A resonantoptical cavity is defined between an external rear grating reflector 64and a selected DBR grating segment 58₁, 58₂, . . . , 58_(N) that dependson the orientation of the external grating reflector 64. Thus, theexternal grating reflector, a collimating and focusing lens 62, thesingle mode waveguide section 56 and the selected DBR grating segment58₁, 58₂, . . . , 58_(N) form a laser oscillator, which is coupled to aflared optical power amplifier 54 to form a broadband tunable MOPAdevice. Whereas the tunability of the MOPA device shown in FIGS. 3A and3B is limited to a relatively narrow range of wavelengths correspondingto the narrow reflection band of the single DBR grating 38, the MOPAdevice in FIG. 4 can be tuned over a broader wavelength rangecorresponding to the stepwise-continuous reflection bands λ₁ ±Δλ₁, λ₂±Δλ₂, . . . , λ_(N) ±Δλ_(N) of the DBR grating segments 58₁, 58₂, . . ., 58_(N) and limited only by the gain band of the diode heterostructure52.

With reference to FIG. 5, an external cavity diode laser has anamplifier chip 66 with a single mode waveguide section 68 coupled to aflared gain section 70. The resonant cavity is defined between anexternal rear reflector 78, here a highly reflective (R≈98%) planarmirror surface, and a front facet 72 of the amplifier chip 66. Rearfacet 74 of the amplifier chip 66 is low reflectivity or antireflection(AR) coated. A lens 76 collimates light emitted from the single modewaveguide 68 through rear facet 74 and focuses reflected light back intothe waveguide 68. In this embodiment, an optical power monitor 80, suchas a silicon photodetector, could be placed behind the mirror surface 78to receive the small amount of light transmitted through the mirrorsurface 78 for monitoring the power level. The detected power levelcould then be used to control the pump current applied to amplifier chip66 in order to maintain relatively stable output powers. Monitoringcould also be used to verify amplitude modulation. A lens system 82 maybe placed in the path of the output beam in front of front facet 72 tocollimate the output beam. Because of the different lightwave beam waistpositions in the lateral and vertical directions, a cylindrical lenssystem may be required.

In FIGS. 6A-6H, eight possible amplifier chip embodiments for use in theexternal-cavity configuration shown in FIGS. 1-5 are depicted. Theamplifier chip 11 seen in FIG. 6A is the same as that shown in FIG. 1and has a single mode waveguide section 17, at a rear end of the chip11, followed by a flared gain section 19 at a front, output, end of thechip 11. The amplifier chip 11 has an antireflection coated or nearly-ARcoated rear facet 23 and a low reflectivity front facet 21. The width ofthe narrow end of the gain section 19 is the same as the width of thewaveguide 17. In FIG. 6B, an amplifier chip 33 also has a single modewaveguide section 35 and a flared gain section 37. However, the narrowend 39 of the gain section 37 has a width W which is not equal to thewidth of the waveguide 35, but is instead wider than the waveguidesection 35. As in the amplifier chip 11 in FIG. 6A, the gain region 37of amplifier 33 is preferably linearly flared and increases at aconstant rate that is slightly greater than the divergence of lightpropagating in the gain region 37. However, gain sections with nonlinearflares, i.e. having increases in width that are not at a constant rateacross the length of the gain section 37 or broad area gain sections,could also be used. The amplifier chip 33 has an antireflection coatedrear facet 43 and a low reflectivity front output facet 41. In FIG. 6C,another amplifier chip 45 has only a flared gain region 47 and no singlemode waveguide section. The flared gain region 47 has a narrow apertureend 49 at the antireflection coated rear facet 51 and a broader outputend at low reflectivity front output facet 53. The flared gain regions19, 37 and 47 in FIGS. 6A-C increase the optical output power whilemaintaining a single spatial mode. Typically, 5 mW cw power at thenarrow input end 29, 39 and 49 of the gain regions are increased over alength of 100 μm or more to greater than 1 W cw output power at theoutput facets 21, 41 and 53. The flared amplifier configurationmaximizes efficiency by expanding the gain volume along the length ofthe amplifier as the optical power grows, so that near uniform powerdensity and saturated carrier density are maintained throughout the gainregion.

In FIG. 6D, an amplifier chip 55 has separate conductive contacts 57 and59 for the single mode waveguide 61 and flared gain section 63. Eachsection 61 and 63 can thus be pumped independently with separateelectrical currents I₁ and I₂. One use of such a configuration is forintensity modulation of the laser. As a result of the individualcontacts 57 and 59, the output power emitted through low reflectivityoutput facet 65 can be modulated by simply modulating the pump currentI₁ supplied to the single mode waveguide section 61, instead of tryingto modulate a single larger pump current provided to the entireamplifier chip. Higher speed modulation and lower modulation currentrequirements are thus achieved with this amplifier chip configuration.The independently pumped single mode waveguide section 61 might also beused as a preamplifier to bring the optical power coupled into it fromthe feedback grating 15 up to saturation levels before the light entersthe flared gain section 63. Also, the single mode waveguide section 61could be used as a phase-control section in which the amount of currentI₁ injected into the phase control waveguide section 61 is adjusted tovary the refractive index in the waveguide and thereby effectivelycontrol the total optical length of the cavity to minimize mode hoppingand extend the tuning range. Such a technique is described by M. Notomiet al. in IEEE Photonics Technology Letters, Vol. 2, No. 2, pages 85-87(1990). More than two separate contacts might also be present on theamplifier chip. For example, the flared gain section 63 could bedifferentially pumped with a lower current density provided by oneconductive contact to the input end 67 of the flared gain section 63 anda higher current density provided by another conductive contact closerto the output end of the gain section 63 near facet 65. Suchdifferential pumping will reduce noise in the optical output signal.Differential pumping with a lower current density in region 67 versusother portions of the flared gain section 63 will also increase thediffraction-limited output power significantly over that obtained from auniformly pumped flared region 63.

Referring to FIG. 6E, differential pumping of an amplifier chip 92 mayalso be achieved by means of a selective proton implantation in the gainregions 84 and 86 during fabrication of the chip 92. The varyingdensities of implanted proton sites in the surface of the amplifier chip92 cause varying resistivities to electric current over the length ofthe gain regions 84 and 86. As a result, a uniform bias voltage appliedto the gain regions 84 and 86 will produce a current densitydistribution that varies in different areas of those gain regions 84 and86, producing differential pumping. In the particular embodiment shownin FIG. 6E, the density of stipling in the drawing represents thesurface conductivity in a particular area of the illustrated amplifierchip 92. Thus it can be seen that the rear portion of the flared gainsection 86 nearest the narrow aperture 90 connecting the flared gainsection 86 to the waveguide section 84 has a lower surface conductivity,and thus is pumped with a lower current density, than the broad frontportion of the flared gain region 86 nearest the low reflectivity frontoutput facet 88 of the amplifier chip 92. The single-mode waveguidesection 84 could have a conductivity which is like the front portion ofthe flared gain region 86 or intermediate or equal or higher in valuebetween the high and low conductivity front and rear portions of theflared gain section 86. Regions near facets 88 and 89 may be leftunpumped to ensure long life at high power, such as in a window laserformed by quantum size effects, impurity induced disordering, doping,composition change or other means.

With reference to FIG. 32, differential pumping can also be done usingan unpumped transparent region 392 as one of the segments of thedifferentially pumped flared region 391. This represents the extremecase in which the pump current density is zero in a first portion 392 ofthe flared region 391. The unpumped transparent region 392 allows thebeam to diverge without forming filaments. Transparency can be achievedfor zero current density by means of quantum size effects, impurityinduced disordering, doping, composition change or other means. Thetransparent region 392 should be proximate to the single mode lightaperture region 393. The end facets 396 and 397 can be AR coated or lowreflectivity coated for a flared optical power amplifier, or HR coatedand low reflectivity coated (about 5% reflectivity) for a flared laseroscillator. Further, if an internal reflector, such as a DBR grating, isformed, the device with transparent unpumped region 392 in the flaredregion 391 could form a MOPA device.

With reference to FIG. 6F, the amplifier chip 100 is essentially thatused in the MOPA device of FIGS. 3A and 3B. The amplifier chip 100includes a single mode waveguide section 102 terminating in a DBRgrating 104 and followed by a flared amplifier section 106. End facets108 and 110 of the chip 100 are antireflection coated.

In FIG. 6G, the amplifier chip 100a also includes a single modewaveguide section 102a followed by a flared section 106a, and AR coatedor low reflectivity facets 108a and 110a. However, here the DBR grating104a is located in the flared section 106a. In a configuration like thatshown in FIGS. 3A and 3B, the laser oscillator will include the portionof the flared section 106a that is located between the single modewaveguide section 102a and the DBR grating 104a, while the optical poweramplifier comprises the remaining portion of flared section 106a betweenthe DBR grating 104a and end facet 110a. The internal reflector 104acould also be an etched mirror.

With reference to FIG. 6H, the amplifier chip 122 has a single modewaveguide 124 that tapers in a section 126 to a smaller aperture leadinginto the flared amplifier section 128 to increase the beam divergence inthe flared region 128. As in previous embodiments, end facets 130 and132 are coated for low reflectivity.

All of the amplifier chips 11, 33, 45, 55, 92, 100, 100a and 122 inFIGS. 6A-6H are light amplifying diode heterostructures with their frontand rear facets suitably coated to prevent self-oscillation. Only whenat least one external reflector, such as the grating 15 in FIG. 1, isprovided to help establish a resonant optical cavity will laseroscillation occur. Alternatively, if the rear facets of each of thesedevices is HR coated devices 11, 33, 45, 55, 88, 92, 100a, and 122 canform unstable resonator lasers which are stable to high coherent powerlevels and do not rely on an external reflector. Tunable gratingsreplacing either reflector can result in broadband wavelength tuning ofsuch a monolithic device. Various heterostructure material compositions,such as GaAs/AlGaAs, InGaAs/AlGaAs, InP/InGaAsP and the like, could beused. Likewise, various known strained, graded index and lattice matchedstructures, as well as various known current, carrier and opticalconfinement structures, including single and multiple quantum wellstructures, may be used. In the case of frequency tunable lasers, suchas the grating tuned laser in FIG. 1, it is desirable to tailor the gainof the amplifier chip so that it remains somewhat constant over a widewavelength range. Such a light amplifying diode heterostructureexhibiting a broadband gain-flattened spectrum can be achieved in singlequantum well structures at high pump current densities, as described byM. Mittelstein, et al. in Applied Physics Letters, 54(12), pp. 1092-1094(1989). The amplifier chip's active region can be optimized for use inexternal cavities like that shown in FIG. 1 by reducing the opticalconfinement Γ in the transverse direction perpendicular to the pnjunction of the chip in order to reduce the transverse or verticaldivergence of light emitted from front and rear facets. In this way, thecoupling efficiency of external optical elements is increased. A furtheradvantage of a lower optical confinement Γ is that lensing associatedwith charge variations and gain saturation is reduced.

With reference to FIGS. 7 and 8, the amplifier chip 71 may have adetuned grating output coupler 73 integrated therein for providingsurface emission 75 of the laser output, rather than end emission from afront facet 77. The detuned grating is located adjacent to the frontfacet 77 at an end of the flared gain section 79 of the amplifier chip71 so that the amplified light propagating in the waveguide defined bythe active region 81 and cladding layers 83 is coupled by the grating 73vertically out of the waveguide and through a top (or bottom) surface 85of the amplifier chip 71. The narrow aperture end 87 of the flared gainsection 79 at the rear end thereof is optically coupled to anantireflection (AR) coated near facet 91, preferably, but notnecessarily, via a single mode waveguide section 89. The overalleffective reflectivity of the grating 73, front facet 77 and outputsurface 85, taken together, is generally low, i.e. less than about 30%,and typically less than about 10%. One result of the grating coupledsurface emission 75 from the amplifier chip 71, when used in a frequencyselective external cavity like that shown in FIG. 1, is that the outputbeam direction is longitudinally steered, forward or backward, as thefrequency is tuned, due to the wavelength dependent nature of thediffraction angle of the grating output coupler 73. Alternatively, frontfacet 77 may be AR coated and grating 73 may be a tuned grating toprovide feedback. In this case, the planar mirror external cavityconfiguration of FIG. 5 is probably preferable and grating 73 may bewavelength tunable.

With reference to FIG. 9, an amplifier chip 120 with a single modewaveguide section 112 and a flared gain section 114 located between apair of parallel planar end facets 116 and 118, of which facet 116 isantireflection coated, has a curved grating output coupled 119 at thebroad end of the flared gain section 114, instead of the straightgrating 73 of FIGS. 7 and 8. Light emerging from the waveguide 112 atthe narrow aperture 117 freely diffracts in the flared gain section 114as a divergent beam. The divergent beam is characterized by curved phasefronts. The grating output coupler 119 is a detuned surface emittinggrating, like the grating 73 in FIGS. 7 and 8, but has a curvature thatmatches the curved phase fronts of the lightwaves propagating in theflared gain section 114. The light emerges through a top or bottomsurface of the amplifier chip 120 as a substantially collimated beam.Single spatial mode filtering by the aperture 117 of the reflected lightreturning to the waveguide 112 works best if the back reflectivity ofthe curved grating 119 is minimized and made substantially less than thelow reflectivity of the planar front facet 118.

Alternatively, an angled external front grating reflector 97, receivinglight emitted through a front end facet 94 of an amplifier chip 93 witha flared gain region, may be used instead of the integral detunedgrating 73 in FIGS. 7-9 to redirect the light into a vertical ortransverse direction, as shown in FIG. 10. If the amplifier chip 93 ispositioned within a frequency selective external cavity having a lens 13and an external rear grating reflector 15, as shown in FIG. 10, then theexternal front grating reflector, outside of the laser cavity defined bythe grating 15 and facet 94, will also steer the direction of thereflected beam 99 longitudinally, that is, forward and backward, as thewavelength of the emitted light is tuned by the orientation of reargrating 15. A collimating lens or lens system 95 may be placed betweenthe front facet 94 of the amplifier chip 93 and the external frontgrating reflector 97, so that the emitted light received by the frontgrating 97 has the same angle of incidence upon the grating 97,regardless of the position of incidence of individual light rays.Alternatively, the grating reflector 97 could be curved to receive thediverging light directly from the emitting facet 94 and reflect it in acollimated beam 99. If longitudinal steering of the beam 99 as thewavelength is tuned is not needed or desired, an angled planar orconcave mirror could be used in place of the front grating reflector 97to simply redirect (or both redirect and collimate) the light output.

With reference to FIG. 11, an amplifier chip 101 is positioned in anexternal cavity having a lens 103 and an external rear grating reflector105. The amplifier chip 101 itself has a single mode waveguide section107 followed by a flared gain section 109. The front facet 111 of theamplifier chip 101 has low reflectivity, establishing a resonant cavitywith the rear grating reflector 105. In this embodiment, the back facet113 of the amplifier chip 101 is formed with a Brewster angle surface115 at least at the aperture of the single mode waveguide section 107.The Brewster angle surface 115 can be formed by ion beam milling and maybe oriented, as shown, with the normal or perpendicular to the surface115 parallel to the pn junction of the amplifier chip 101.Alternatively, the normal to surface 115 could be perpendicular to thepn junction. The orientation in which the Brewster angle surface 115 isformed determines whether the TE or TM polarization mode of oscillationis supported by the cavity. The Brewster surface 115 also increases thecontinuous tuning capabilities of the external cavity by directing anylight reflected by the surface 115 out of the single mode waveguide 107,thereby effectively minimizing back facet reflectivity and pre- ventingself-oscillation of the amplifier chip 101. The facet 113 and Brewstersurface 115 might additionally be antireflection coated.

With reference to FIG. 12, another amplifier chip 121, located in anexternal cavity with a lens 123 and external grating reflector 125, hasa single mode waveguide section 127 and a flared gain section 129 as inany of FIGS. 6A-6H and 7. A front end output facet 131 is of lowreflectivity and defines a resonant cavity along with the externalgrating reflector 125. The orientation of the grating 125 can beadjusted to select the frequency of laser oscillation. The amplifierchip 121 also has a back facet 133 that is tilted at an angle, typicallyabout 45°, sufficient to cause it to be totally internally reflecting ofbackward propagating light in the waveguide and also of light fed backby grating reflector 125 and focused by lens 123 onto the facet 133 inthe neighborhood of the single mode waveguide section 127. Light is thuscoupled vertically through a substantially nonreflective top (or bottom)amplifier chip surface 135. Surface 135 can be AR coated also. Use ofthe totally internally reflecting angled back facet 133 minimizesself-oscillation, because backward propagating light from the waveguidesection 127 is reflected vertically out of the waveguide and allowed tofreely diffract in the amplifier chip 121 before reaching the outputsurface 135. Any light reflected by the substantially nonreflectiveoutput surface 135 has little chance of being coupled back into thewaveguide section 127.

With reference to FIGS. 13 and 14, in order to reduce the alignmentsensitivity of the external cavity laser, the back facet 143 of theamplifier chip 141 can be coated such that a top portion of the facet143, including the light emitting aperture 145, is antireflective, whilea bottom portion of the facet 143 is highly reflective. The amplifierchip 141 is otherwise like any of those shown in FIGS. 6A-H, with anoptional single mode waveguide section 147 and with a flared gainsection 149 in which light propagates in a waveguide 151 and isamplified. The front light emitting end facet 153 has low reflectivityand forms, along with an external rear reflector 157, an externalresonant cavity. Rear reflector 157 can be a planar mirror or a gratingreflector. A lens 155 is positioned between the rear facet 143 of theamplifier chip 141 and the external rear reflector 157 to collimate thelight emitted from the narrow waveguide section 147 through the aperture145 and to bend the light path slightly downward. This light isreflected by the rear reflector 157 back toward the lens 155, which thenfocuses the light onto the HR coated bottom portion of rear facet 143.The light is then reflected from the HR coating back toward the lens155, where it is collimated and bent onto a slightly upward return path.Being reflected a second time from the external rear reflector 157, thelight is finally focussed by the lens 155 back onto the aperture 145 andcoupled into the waveguide 147. In this way, difficult alignmenttolerances in the direction perpendicular to the pn junction arereduced, because the vertical alignment of the amplifier chip 141 withthe lens 155 and external rear reflector 157 only determines the amountof light path bending by the lens 155, and does not adversely affect thecoupling of light back through the aperture 145. In place of the lens155 and planar rear reflector 157, a suitably curved external rearreflector could be used, as described in U.S. Pat. No. 4,797,894 toYaeli. In the case where the external rear reflector 157 is a frequencyselective grating reflector, as seen in FIG. 1, the reflection of lighttwice off of the rear reflector 157 provides an additional advantage.Due to two pass operation of the grating reflector 157, the spectralline width of the light coupled back into the aperture 145 is reducedsubstantially.

With reference to FIGS. 15 and 16, an amplifier chip 161 may have amonolithic array of sources 162, 163, etc. for simultaneous operation inmultiple wavelengths λ₁,λ₂, etc. Each source 162, 163, etc. on theamplifier chip 161 is constructed, as in FIGS. 6A-H and 7, with a flaredgain section 164 and a spatial mode filter, such as a single modewaveguide section 165, at the narrow end of the flared gain section 164.The front output facet 167 at the broad end of the flared gain section164 is characterized by low reflectivity and forms, together with anexternal rear grating reflector 173, a resonant cavity for laseroperation. The rear end facet 169 is antireflection (AR) coated toprevent self-oscillation of the amplifier chip 161. The flared gainregions 164 of each source 162, 163, etc. in the array can be fabricatedto amplify at different emission wavelengths. This is done, for example,by using a single quantum well strained layer InGaAs/InAlGaAs laser,where the gain band may be 50 nm wide. In order to make the singlemonolithic amplifier chip 161 capable of operating over a largewavelength range (e.g. 630 nm to 1100 nm) with each array element 162,163, etc. operating over about a 50 nm bandwidth, the multiwavelengthamplifier array can be fabricated as described for laser arrays in U.S.Pat. Nos. 4,925,811, 4,955,030 and 5,039,627 to Menigaux et al. Theselaser array structures have stacks of alternate confinement layers andactive layers, with each active layer being of a different compositionfrom the others and being characterized by a different gain wavelength.PN junctions are formed in the vicinity of different active layers inthe stack by means of localized introduction and diffusion of a p-typeimpurity to different depths. An alternative way to form amultiwavelength amplifier array is to use multiple amplifier chips, eachhaving a different amplifying wavelength. In either case, light beamsemitted through the apertures 170 of the multiple sources 162, 163, etc.at the AR coated rear facet 169 are collimated by a lens 171 andreflected from the external grating reflector 173 back through the lens171 to be focussed on the rear facet 169. The relative positions of theapertures 170 with respect to the lens 171 determine the amount ofbending of the light paths for the various emitted beams, and thereforedetermine the different incidence angles of each beam on the grating173. Only light of particular wavelengths λ₁, λ₂, etc. corresponding tothe respective incidence angles are coupled back through the apertures170 into the amplifier chip 161. Thus, each source 162, 163, etc.located in the external cavity will only oscillate at a particularwavelength λ₁, λ₂, etc. corresponding to the incidence angle of lightfrom that source onto the grating 173. The gain band of the activemedium for each source 162, 163, etc. should be selected to match itsresonance band in the cavity. A Fabry-Perot external mirror could beused in place of grating reflector 173.

With reference to FIG. 17, several single mode waveguide regions 175 and177 may be coupled to a single flared gain region 179 on an amplifierchip 181 for providing a wavelength switching capability. As in FIG. 16,the amplifier chip 181 has a front light emitting facet 183 that is oflow reflectivity and that forms, along with external rear gratingreflector 189, a resonant cavity for laser oscillation. Rear facet 185of the amplifier chip 181 is antireflection coated. A lens 187 ispositioned between the rear facet 185 and the grating reflector 189. Asin FIG. 16, the difference in position of the light emitting waveguideapertures 175 and 177 at rear facet 185 relative to the lens 187 resultsin a difference in incidence angle of the light beams upon the grating189 and thus a difference in the wavelength λ₁ or λ₂ that can oscillate.The waveguides 175 and 177 should be spaced sufficiently far apart tominimize crosstalk, but sufficiently close that both wavelengths λ₁ andλ₂ fall within the gain spectrum of the common flared gain region 179.Separate electrical contacts 191 and 193 independently bias thewaveguides 175 and 177 and independently inject current I₁ and I₂ intothe respective waveguides. At least one other conductive contact 195provides current I₃ to the flared gain region 179. Biasing only one ofthe waveguides 175 and 177 so as to minimize loss in the selectedwaveguide will select the wavelength λ₁, λ₂ of the output beam.Alternatively, operating both waveguides 175 and 177 would lead tosimultaneous multiple wavelength operation, as well as the generation ofharmonics of the wavelengths λ₁ and λ₂.

With reference to FIGS. 18-28, other cavity configurations using one ofthe above described amplifier chips with flared gain region arepossible. For example, a saturable absorber 205 can be incorporated intothe external cavity to act as a Q-switch for generating short pulses, asseen in FIG. 18. An amplifier chip 201 having a flared gain region 202is placed within a resonant cavity defined by a low reflectivity frontend facet 209 of the amplifier chip 201 and an external rear reflector207. Rear reflector 207 may be a mirror surface or a grating reflector.A lens 203 and the saturable absorber 205 are positioned between an ARcoated rear facet 204 of the amplifier chip 201 and the rear reflector207. Lens 203 receives light emitted from the narrow aperture end of theflared gain region 202 through AR coated rear facet 204 and collimatesthe light beam for passage through saturable absorber 205 to the rearreflector 207. Lens 203 also receives the return light reflected fromthe rear reflector 207 back through saturable absorber 205 and focusesthe light onto the rear facet 204 for coupling through the narrowaperture into the flared gain region 202 of the amplifier chip 201. Thecavity configuration provides for mode locked, high average and peakpower operation of laser in a pulsed mode. A saturable absorber formedby an unpumped or reverse biased region in the chip along the length ofthe cavity can also provide mode locked operation. Synchronous pumpingof at least single mode gain region 202 or single mode gain region 206at a period coinciding with the round trip transit time of light pulsesin the cavity also results in picosecond pulse lengths and very highpeak power outputs. The effectively unstable resonant cavity provided inpart by the flared gain region 202 together with narrow, single mode,aperture 206 ensures stable, single mode operation even at high power.

As seen in FIGS. 19 and 20, the amplifier chips 211 and 221 may beoptically coupled to an external optical fiber 213 and 225. The opticalfibers 213 and 225 are preferably single mode fibers. In FIG. 19, thefiber 213 is butted to the AR coated rear facet 214 of the amplifierchip 211 aligned with the position of the narrow single mode aperture ofthe flared gain region 212 of the amplifier chip 211. In FIG. 20, thefiber 225 is coupled via a lens 223 to receive the light emitted fromamplifier chip 221 through the AR coated facet 224. Lens 223 may be agraded-index (GRIN) rod lens, as shown. In FIG. 19, feedback is providedby a highly reflective coating 215 on the far end of the optical fiber213 so that a resonant cavity for laser oscillation is establishedbetween the reflective coating 215 and the low reflectivity front endfacet 217 of the amplifier chip 211. In FIG. 20, frequency selectivefeedback is provided by a tunable Bragg grating reflector 227 associatedwith the optical fiber 225. The fiber 225 might be held in a groove 223formed in a support 226 and the grating reflector 227 placed over thefiber 225 on the support 226. The end of the fiber 225 beyond thegrating reflector 227 can be cleaved at an angle to minimize feedbackfrom end reflections. The grating reflector 227 has a refractive indexwhich is higher than the fiber's cladding so that coupling of light isvia the evanescent wave leaking out of the cladding into the gratingreflector 227 above it. The fiber cladding may need to be thinned inthis region for adequate coupling to occur. If the grating teeth arearranged in a fan shape, as shown, tuning can take place by sliding thereflector 227 from side-to-side to adjust the grating pitch in thevicinity of the fiber 225. W. Sorin, et al. reported in Optics Letters,Vol. 13, No. 9, pages 731-733 (1988) that a laser diode coupled to suchan external fiber cavity is stepwise tunable over about a 66 nm range.In combination with our amplifier chip 221 with flared gain region 222,the fiber coupled laser achieves stable, high power single frequencyoperation with a narrow line-width. A grating could also be formedwithin the optical fiber itself and be tunable by applying stress to thefiber in the grating region.

In FIG. 21, an atomic resonance filter 235 is incorporated into theexternal cavity. The arrangement is similar to that shown in FIG. 18,but with the atomic resonance filter 235 replacing the saturableabsorber 205. The cavity is defined by an external rear reflector 237,such as a mirror or grating, and a low reflectivity front end facet 239of an amplifier chip 231. The amplifier chip 231 may be any of thoseshown in FIGS. 6A-6H, 7, 9, 11, 12, 13, 15, 16, 17, 24, 25, 26, 28 and37 described herein. A lens 233 between the amplifier chip 231 and theatomic resonance filter 235 collimates light received from the amplifierchip 231 and focuses return light reflected from the rear reflector 237so as to couple the light back into the amplifier chip 231. Theresulting laser produces a stable, single frequency output 240 having afrequency that is referenced to an atomic resonance frequency specifiedby the filter 235.

In FIG. 22, the external resonant cavity, defined by an external rearreflector 247 and a low reflectivity front end facet 249 of an amplifierchip 241 with flared gain region 242, includes a birefringence filter245 for wavelength selection. Such a birefringence filter 245 could betunable. For example, as described by A. Schremer, et al. in AppliedPhysics Letters 55(1), pages 19-21 (1989), an electrooptic birefringentmodulator can be placed in external cavities for wavelength tuning.Frequency modulation of the laser output could also be performed usingsuch a configuration. The amplifier chip 241 with flared gain region 242of the present invention ensures high power, single spatial mode outputsas the frequency is tuned or modulated.

In FIG. 23, instead of the rear grating reflector 15 of FIG. 1, a prism255 and a mirror 257 could be used to provide wavelength selectivefeedback in the external cavity and to obtain a frequency tunableoutput. An amplifier chip 251 has a flared gain region 252, as in FIGS.2A-2D. A rear end facet 254 is antireflection coated to preventself-oscillation, while a front end facet 259 has low reflectivity.Together, the external mirror 257 and front end facet 259 define aresonant optical cavity for laser oscillation. A lens 253 receives andcollimates light emitted from the narrow aperture end of the flared gainregion 252 at the AR coated facet 254. A prism 255 is positioned betweenthe lens 253 and mirror 257 and oriented for refracting the collimatedbeam received from lens 253. Preferably, prism 255 is made of amaterial, such as dense flint glass, that has high dispersion |Δλ/Δθ| inthe wavelength band coinciding with the gain band of the amplifier chip251. The orientation of the mirror 257 determines which wavelength oflight will be incident perpendicular thereon and therefore whichwavelength will be fed back into the gain region 252.

With reference to FIG. 24, the single mode waveguide 263 serving as aspatial mode filter in amplifier chip 260 need not extend all of the wayto the antireflection (AR) coated intracavity rear facet 267. Rather, ashort flared region 265 could be provided in the amplifier chip 260between the single mode waveguide 263 and the rear facet 267. As inprevious laser embodiments, the resonant cavity is defined between anexternal rear grating reflector 274 and a low reflectivity front facet269 of the amplifier chip 269. In operation, light that has theparticular wavelength to be reflected by the grating reflector 274 backonto its incident light path will be focussed by lens or lens system 272through the rear facet 267 into the short flared region 265. The shortflared region 265 tapers down to the single mode waveguide 263 at a ratethat allows the light to be efficiently coupled into the waveguide 263.Forward propagating light emerging from the single mode waveguide 263 isthen allowed to freely diffract and diverge in the principal flared gain261 of the amplifier chip 260, in which the optical power is increasedto high levels at the output facet 269.

With reference to FIGS. 25A and 25B, a light amplifying diodeheterostructure 280 with a pn rectifying junction 282 for providing anactive region for amplification of lightwaves has a single modewaveguide section 284 for guiding propagation of the lightwaves and atwo-part flared gain section 286a and 286b in which the lightwaves areallowed to freely diverge as they are amplified. In this embodiment,proton surface implants 288 electrically isolate the respectivewaveguide section 284 and flared gain section parts 286a and 286b sothat each isolated part can be separately pumped with a different amountof electric current I₁, I₂ and I₃. Other electrical isolationtechniques, such as selective surface etching, could be used in place ofproton surface implants 288 to provide the isolation. Separateconductive surface contacts for each region apply the different currentlevels I₁, I₂ and I₃ to the respective regions. Current I₁ applied tosingle mode waveguide 284 may be adjusted to optimally excite the narrowaperture end of flared gain region 286a and/or can be modulated tomodulate the laser output. Current I₂ applied to the narrower rear part286a of the flared gain region may be lower than the current I₃ appliedto the broader part 286b in order to minimize amplification of signalnoise and to suppress the formation of filaments. Current I₂ could alsobe modulated to modulate the laser output. The laser's resonant cavityis again defined by an external rear grating reflector 294 for frequencytunability and by a low reflectivity front output facet 292 of theamplifier chip 280. Alternatively, the grating reflector 294 may bereplaced by a plane mirror. In this case, multiple longitudinal modeoperation is possible and mode locking of the output can be achieved bymodulation of at least a portion of the single mode waveguide 284 (orother regions) with a current at a frequency of c/2 nL, where c/n is thespeed of light in the cavity and L is the cavity length. Also, note thatpassive mode locking could also be used. A reverse bias section ofwaveguide 284 could provide a saturable absorbing region, for example.Further, note that mode locking is possible, if external reflector 294were replaced by a reflective surface for facet 290, thus making thedevice entirely monolithic. A lens or lens system 296 collimates lightemitted from the single mode waveguide 284 through antireflection coatedrear facet 290 for reflection by the grating 294 and focuses the lightreflected by the grating 294 back onto rear facet 290 for coupling intowaveguide 284.

With reference to FIG. 26, a MOPA device 301 includes an amplifier chip303 with antireflection (AR) coated end facets, an external rearreflector 305 and a collimating and focusing lens 307. The amplifierchip 303 includes a single mode waveguide section 309 to which a firstcurrent I₁ is applied, distributed Bragg reflector (DBR) grating 311 atan end of the waveguide 309 to which a tuning current I_(t) or a tuningbias voltage V_(t) may be applied, and a two-part flared amplifiersection 313a and 313b optically coupled via the grating 311 to thewaveguide 309 to which respecting amplifying currents I2 and I3 can beapplied. The resonant optical cavity of the MOPA's laser oscillator isdefined by external rear reflector 305 and the wavelength tunablegrating 311 integrated in the chip 303. This embodiment, thus, can tunethe wavelength of the MOPA device's light output beam by changing thetuning bias V_(t) or current I_(t) that is applied to the integral DBRgrating 311. In operation, wavelength tuning may or may not be desired.Currents I₂ and I₃ may be differentially pumped with a lower currentdensity under I₂ to provide high coherent output power. A differentseries resistance in these regions could provide the same benefit.Tailoring the chip's internal resistance, as in FIG. 6E, could allow I₁,I₂ and I₃ to all be driven from the same electrical contact. Current I₁may also be used for phase control tuning between mirror 305 and grating311.

Each of the above-described lasers and MOPA devices is characterized bya light amplifying diode heterostructure or amplifier chip with a flaredgain region that allows light propagating therein to freely diffract. Inthe lasers of the present invention, the diverging light which ispartially reflected by the low reflectivity front end facet continues todiverge, so that only the central rays pass through the narrow apertureend of the flared gain region and through the antireflection coated rearfacet of the amplifier chip to the external rear reflector of theresonant cavity. Effectively, the cavity acts as a marginally stableresonator with a single mode spatial filter that ensures a singlespatial mode of oscillation. The light output from the broad front endof the flared gain region is characterized by high powers (in excess of1 W cw) and good modal quality. Embodiments with frequency selectiveelements in the external cavity are tunable over a bandwidth of at least50 nm and provide stable, single frequency and narrow linewidth outputseven at high output power levels.

With reference to FIG. 27, a laser made up of a light amplifyingsemiconductor diode heterostructure 321 includes a pair of reflectioncleaved end facets 323 and 325 integral with the heterostructure 321 forproviding feedback of light and for defining an optical resonant cavityfor laser oscillation. The heterostructure 321 has a multimode gainregion 327, preferably flared, permitting the propagation of lighttherein with a diverging phase front. The flared gain region 327 has asingle spatial mode light aperture 329 at a narrow end of the flaredgain region 327. The aperture 329 preferably comprises a single modewaveguide section 331 in the diode heterostructure 321 optically coupledat one end to the flared gain region 327. The opposite end of thewaveguide 331 terminates in the rear end facet 323, which is preferablycoated for high reflectivity. The front end facet 325 at the broad endof the flared gain region 327 furthest from the single mode aperture 329has a low reflectivity of at least about 0.5%, but preferably not morethan about 5%. A reflectivity of about 1% is typical. The diverginglight that is reflected by front end facet 325 continues to diverge.Only the central-most rays couple through the narrow aperture 329 intothe waveguide 331 to be reflected by the rear facet 323. Lossy regions333 may be provided at the side edges of the waveguide 331 and gainregion 327 near the aperture 329 in order to suppress oscillation of anylight that could be coupled out of the gain region 327 into otherportions of the diode heterostructure 321 other than through the lightaperture 329. The lossy regions 333 may be low bandgap absorptionregions formed by impurity induced disordering or implantation or byepitaxial growth of different levels or heights, such as by growth upona terraced or channeled substrate. Alternatively, the topheterostructure layers could be etched away in these regions 333 downthrough the active layer or layers. Laser light 335 is emitted throughthe low reflectivity front end facet 335, where it could be collimatedby an external lens system, not shown.

Referring now to FIGS. 28 and 29, one of the reflective cleaved endfacets 323 or 325 of FIG. 27 may be replaced by a grating reflector 335or 337 at one end of the diode heterostructure 341 or 351. In FIG. 28,the grating reflector 335 is located at the broad end of the flared gainregion 339. The end facet 343 adjacent to grating reflector 335 isantireflection coated to suppress Fabry-Perot cavity modes. Differentialpumping of flared gain region 339 is provided by creating resistiveregions R by ion implantation or other means. In FIG. 29, the gratingreflector 337 is located at the rear end of single mode waveguidesection 353. The rear end facet 355 proximate to grating reflector 337may be oriented at a nonperpendicular angle θ relative to the principaldirection of light propagation in waveguide section 353 and flared gainregion 357 in order to suppress reflection from this surface. Thegrating reflectors 335 and 337 provide single frequency reflection oflightwaves in the resonant cavity and can be tuned by a bias voltageV_(tun) or tuning current I_(tun) applied to a conductive contact abovethe gratings 335 and 337 so as to adjust the wavelength reflectionresponse of the gratings 335 and 337. In addition to a pump currentI_(g) applied to single mode waveguide sections 345 and 353, a separatephasing current I.sub.φ may also be applied to an area of the waveguidesections 345 and 353 to adjust the optical path length of the resonantcavity to match the phase of the light propagating in the cavity to theselected wavelength. This enables continuous wavelength tuning byadjusting the tuning voltage or current V_(tun) or I_(tun) and thephasing current I.sub.φ in concert to prevent or minimize longitudinalmode hopping as tuning takes place. The flared gain regions 339 and 359can be differentially pumped along their lengths either by applying asingle amplification current I_(amp) and providing surface resistiveregions R in the narrower area of the gain region 339, as in FIG. 28, orby applying separate amplification currents I_(amp1) and I_(amp2) torespective narrower and broader areas of the flared gain region 357, asseen in FIG. 29.

With reference to FIG. 30, a flared laser oscillator 361 includes twosingle spatial mode waveguide sections 363 and 365 connected to a narrowend of a common flared gain region 367. The resonant cavity is definedbetween reflective cleaved end facets 369 and 371. Wavelength tuning andfar field beam steering can be accomplished by changing the current I₁and I₂ applied to the waveguide sections 363 and 365. If differentcurrents I₁ and I₂ are applied simultaneously, the laser output is oftunable wavelength. As in previous embodiments the flared gain region367 can be differentially pumped along its length with separate appliedcurrents I₃ and I₄.

Referring to FIG. 31, another flared laser oscillator 373 also hasmultiple single mode waveguide sections 375, 377 and 379 feeding througha single mode combining section 381 to a common flared gain region 383.However, grating reflectors 385-387 having grating pitches Λ₁, Λ₂ and Λ₃and placed in the light path, here at the rear ends of the single modewaveguide sections 375, 377 and 379, are used to select the wavelengthsof the laser output. The resonant cavity is defined by the gratings385-387 and the low reflectivity front end facet 389. The rear facet 390may be oriented at a nonperpendicular angle to the direction of lightpropagation in order to suppress possible Fabry-Perot modes ofoscillation. Different currents I₁ -I₅ may be applied to respectivesections 375, 377, 379, 381 and 383. Excitation of the various sectionswith currents of I₁, I₂ or I₃ also results in discrete switching ofoutput wavelengths.

We claim:
 1. A semiconductor laser source comprising:a longitudinallyextending body of semiconductor material having an optical cavity, adiverging gain region included in a portion of the body longitudinalextent and having a narrow input end and wider output end for achievinghigher power output, a single spatial mode aperture region included inanother portion of the body longitudinal extent optically coupled tosaid diverging gain region narrow input end, a portion of said singlespatial mode aperture region for mode-locking the operation of saidlaser source, and means to modulate said portion of said single spatialmode aperture region independent of electrical operation of saiddiverging gain region to achieve mode lock operation of said lasersource.
 2. The laser source of claim 1 wherein at least a portion ofsaid diverging gain region is modulated independently of and in lieu ofelectrical operation of said single spatial mode aperture region toachieve mode lock operation of said laser source.
 3. The laser source ofclaim 1 wherein said mode lock operation is achieved by modulation of atleast a portion of said single spatial mode aperture region with acurrent at a frequency of c/2 NL where c/n is the speed of light in saidoptical cavity and L is optical cavity length.
 4. A semiconductor lasersource comprising:a longitudinally extending body of semiconductormaterial having an optical cavity, a diverging gain region included in aportion of the body longitudinal extent and having a narrow input endand wider output end for achieving higher power output, a single spatialmode aperture region included in another portion of the bodylongitudinal extent optically coupled to said diverging gain regionnarrow input end, means to independently pump said diverging gain andsingle spatial mode aperture regions, a saturable absorbing regionformed within a portion of said single spatial mode aperture region formode-locking the operation of said laser source, and means to apply areverse bias to said saturable absorbing region independently of saidother regions.
 5. The laser source as in claim 1 or 4 wherein saidsemiconductor source is an unstable resonator.
 6. The laser source as inclaim 1 or 4, wherein said semiconductor source is a stable resonator.7. The laser source of claim 4 wherein said gain region, single modewaveguide region and saturable absorbing region comprise at least aportion of an optical cavity of said laser, feedback means is includedin said optical cavity comprising a mirror facet on a side of saidsingle mode waveguide region opposite to said coupled gain region narrowaperture.
 8. The laser source of claim 4 wherein said gain region,single mode waveguide region and saturable absorbing region comprise atleast a portion of said optical cavity of said laser, feedback means isincluded in said optical cavity comprising an external mirror adjacentto an antireflecting coated facet on a side of said single modewaveguide region opposite to said coupled gain region narrow aperture.9. A semiconductor laser source comprising:a longitudinally extendingbody of semiconductor material, a diverging gain region included in aportion of the body longitudinal extent and having a narrow input endand wider output end for achieving higher power output, a single spatialaperture region included in another portion of the body longitudinalextent optically coupled to said diverging gain region narrow input end,and means within said single spatial mode aperture region to mode lockthe operation of said laser source, said diverging gain regioncomprising adjacently disposed diverging gain sections, means toelectrically isolate and independently pump said adjacently disposeddiverging sections.
 10. The laser source of claim 9 wherein saidelectrically isolated diverging gain sections are current pumped, saiddiverging gain section including said gain region narrow input endpumped with a pumping current lower than the pumping current for anotherof said diverging gain sections.
 11. A semiconductor laser sourcecomprising:a longitudinally extending body of semiconductor material, adiverging gain region included in a portion of the body longitudinalextent and having a narrow input end and wider output end for achievinghigher power output, a single spatial mode aperture region included inanother portion of the body longitudinal extent optically coupled tosaid diverging gain region narrow input end, means to independentlydrive said single spatial mode aperture region to provide an opticaloscillator and said diverging gain region to provide high power output,an optical cavity formed longitudinally in said coupled gain region andsingle spatial mode aperture region and extending externally of saidsemiconductor body to optical feedback means for said opticaloscillator, and saturable absorber independent of said semiconductorbody positioned within a portion of said optical cavity which isexternal of said semiconductor body, operation of said saturableabsorber for mode locking operation of said laser source via pumping ofeither at least a portion of said diverging gain region or said singlespatial mode aperture region at a period coinciding with the round triptransit time of light pulses in said optical cavity.
 12. The lasersource of claim 11 means to reverse biased said saturable absorbingregion.
 13. The laser source as in claim 11, wherein said semiconductorsource is an unstable resonator.
 14. The laser source as in claim 11,wherein said semiconductor source is a stable resonator.
 15. The lasersource of claim 11 wherein said mode lock operation is achieved bymodulation of at least a portion of said single spatial mode apertureregion with a current at a frequency of c/2 nL where c/n is the speed oflight in said optical cavity and L is optical cavity length.